A wide variety of intravascular devices are used in various medical procedures. Certain intravascular devices, such as balloon catheters, diagnostic catheters, stent delivery catheters, and guidewires are generally used simply to deliver fluids or other medical devices to specific locations within a patient's body, such as a selective site within the vascular system. Other, frequently more complex, devices are used in treating specific conditions, such as devices used in removing vascular occlusions or for treating septal defects and the like.
In certain circumstances, it may be necessary to occlude a patient's vessel, chamber, channel, hole, or cavity such as to stop blood flow there through. In other cases it may be necessary to create a flow restriction or to shunt flow from one vessel to another to treat abnormal cardiovascular conditions. Examples of selective occlusion are, without limitation, closure of a Patent Ductus Arteriosus (PDA), Atrial Septal Defect (ASD), Ventricular Septal Defect (VSD), Patent Foreman Ovale (PFO), Arterial Venous Fistula (AVF), or an Arterial Venous Malformation (AVM).
Mechanical embolization devices are well known in the art and sold commercially for occlusion of vessels in various locations within the vasculature. Intravascular occlusion devices can be fabricated from Nitinol (NiTi) wire strands that have been braided to form a tubular fabric which is then heat set in a mold to an expanded shape, but which can be compressed for delivery through a catheter to a treatment site whereby the device, when urged out of the delivery catheter, self-expands within the vasculature to occlude blood flow at the treatment site. The details of the various designs and configurations as well as methods of fabricating and using the devices are known in the art.
An example of a shunting procedure is shunting of blood between the portal vein and the hepatic vein; know as a Transjugular Intrahepatic Portosystemic Shunt (TIPS). Certain forms of congenital disease may require a communication between the right atrium and left atrium. Shunting may also be required for treating specific abnormal conditions, such as bi-passing vascular occlusions within an internal passageway.
Congenital heart defects are examples of the necessity for flow restriction where holes in the septum allow blood to flow from the high pressure left ventricle to the lower pressure right ventricle causing excess blood flow to the lungs. The body's natural reaction is to constrict the vessels to the lungs to restrict blood flow. Over time, this causes a thickening of the pulmonary arteries and ultimately leads to closure of smaller lung arteries and further complications if left untreated. The treatment involves early mechanical flow restriction of blood to the lungs until a surgical fix can be accomplished.
The occluding, shunting, and flow restricting devices described above use similar technology for fabrication. Each device is formed from a plurality of resilient metal strands of a shape memory alloy woven into a braided fabric to create a resilient material which can be heat treated to substantially set a desired shape. In performing the heat treatment step, the braided fabric is first deformed to generally conform to a molding surface of a molding element and the braided fabric is then heat treated in contact with the surface of the molding element at an elevated temperature. The time and temperature of the heat treatment is selected to substantially set the braided fabric in its deformed state. After the heat treatment, the fabric is removed from contact with the molding element and will substantially retain its shape in the deformed state. The braided fabric so treated defines an expanded state of a medical device, but which can be longitudinally stretched to reduce its cross-sectional profile so that it can be deployed through a catheter into a channel in a patient's body. The device connects to a delivery device by a threaded connection. Once the delivery catheter's distal end with the device contained within its lumen is placed at the treatment site, the device is urged out of the delivery catheter and self-expands to its expanded preset configuration. Once the device is positioned as desired, the delivery device is unthreaded and the delivery catheter and delivery device are removed from the body.
One limitation of these devices is the need to clamp the ends of the wire strands at each end of the device to prevent unraveling. In such untreated NiTi fabrics, the strands will tend to return to their unbraided configuration and the braid can unravel fairly quickly unless the ends of the length of braid that has been cut to form the device, are constrained relative to one another. One method which has proven to be useful to prevent the braid from unraveling is to clamp the braid at two locations and cut the braid to leave a length of the braid having clamps at either end, thereby effectively defining an empty space within a sealed length of fabric. These clamps will hold the ends of the cut braid together and prevent the braid from unraveling.
Alternatively, one can solder, braze, weld or otherwise affix the ends of the desired length together (e.g., with a biocompatible cementitious organic material) before cutting the braid. Although soldering and brazing of NiTi alloys have proven to be fairly difficult, the ends can be welded together, such as by spot welding with a laser welder.
Devices marketed using these technologies include the braided metal clamps to prevent unraveling of the metal strands. The clamps add to the diameter of the collapsed device for delivery through a catheter as well as project outward from some configurations of the device. These outward projections are often in the blood flow path and could be a source of clot formation or result in flow disruption.
Some have provided a recess in each end surface of the device where each braided end of the device is held together with a clamp. The clamps are recessed into the expanded diameter portion of the device, thereby reducing the overall length dimension of the device and creating a low profile occluder. However, the recessed clamps cause the fabric to reverse direction in the heat-set state. In the compressed state, the wires are higher stressed and exert an increased outward drag against the wall of the delivery catheter making it more difficult to push the device through the catheter.
In the case of a flow restrictor or shunt device, the braided wire end clamps make the device configuration bulky and un-necessarily complex, since the natural placement of the clamps is in a co-axial position to the braided tube, which ideally, is where the flow path should be. The designs described require extra manufacturing steps to create the flow path. In addition the manufacturing cost of the device is higher than need be if the clamps were not used.
With reference to FIGS. 1A-C, 2, 3, and 4, prior occluders, shunts, and flow restrictors are shown respectively. FIGS. 1A-C illustrate an occluder design that may be described as having a flanged or disc shape at each end, connected by a smaller diameter portion between them. FIGS. 2 & 3 are two views of a shunt device. FIG. 4 is an example of a flow restrictor.
FIG. 1A shows the design of an occluder 10 having enlarged diameter discs or flanges 11 & 12 at each end and a small connecting diameter between the ends. Each end of the device has a wire end clamp. The distal clamp 14 and proximal clamp 13 hold the wire ends from unraveling. The side view FIG. 1B illustrates how clamps 13 extend from the end of the device. Clamp 13 contains internal threads 15 that mate with external threads 16 on delivery device 17 as depicted in FIG. 1C. A polyester fabric disc 18 is used to improve device thrombogenicity and is sutured into disc 12. The fabric collapses with the device for delivery through a delivery catheter.
It would be desirable for a medical device to achieve occlusion, flow restriction, or shunting of blood in the human vasculature that is:
of a lower collapsed deliverable profile;
deliverable through a delivery catheter with less force;
less intensive to manufacture;
less disruptive to blood flow; and
can be manufactured at a reduced cost.